Backwoodsman, the rule for doubling a final consonant goes like this:
When adding a suffix that begins with a vowel to a one-syllable word which ends in a single consonant preceded by a single vowel, (or a multi-syllable word, the last syllable of which is accented), double the final consonant.
The purpose of doubling the final consonant is to avoid turning the preceding vowel from a short to a long vowel sound.
In this case we're talking about a word (summit) a word the last syllable of which is NOT accented, "SUM-it". Adding "ed" or "ing" does not turn the "i" from "ih" to "eye".
Now if the word in question were "submit", the rule would apply differently because it is pronounced "sub-MIT", so to avoid changing the "ih" sound to "eye", as in "mite", we double the "t" in "submit".